1. Field of the Invention
The invention relates to a touch input device and a touch sensor circuit, and more particularly to a touch input device and a touch sensor circuit with differential detection.
2. Description of the Prior Art
With the development of information technology, the electronic products become more diversified and humanized. For example, based on the touch panel or touch pad nowadays, user may simply operate the device or type some sentences with their finger or a touch pen, instead of using the mouse or keyboard to type in the traditional and inconvenient way.
There are a number of types of touch panel technology: resistive type, capacitive type, optical type and surface acoustic wave type. The resistive-typed touch panel mainly includes upper and lower ITO glass layers. When an object touches the upper ITO glass layer and presses it toward the lower ITO glass layer, the controller of the touch panel will generate a voltage signal, and coordinates of the contact point can be computed according to the voltage signal.
The capacitive-typed touch panel has two types: surface capacitive touch panel and projected capacitive touch panel. The surface capacitive touch panel includes a piece of conductive glass. Two surfaces of the conductive glass are coated with conductive material. External surfaces of the conductive glass are further coated with a protective film. The electrodes around the glass plate builds up an electric flied on the glass surfaces. When users touch the touch panel with their fingers, the fingers will be coupled to the capacitance on the glass surface and induce a small current. The controller of the touch panel may compute the coordinates of the contact point according to the current.
Recently, the projected capacitive touch panels are widely applied in various touch-input electronic devices (e.g. smart phones). The locating theory of the projected capacitive touch panel is based on the capacitive variation of the sensor grid implemented within the touch panel. Please refer to FIG. 1, FIG. 1 is a schematic diagram illustrating a touch panel 10 in prior art. As shown in FIG. 1, there are multiple X-directional conductive lines (X1˜Xm) and multiple Y-directional conductive lines (Y1˜Yn) disposed on different layers in the touch panel 10. The X-directional conductive lines and the Y-directional conductive lines are cross to form the sensor grid. Each cross point of one X-directional conductive line and Y-directional conductive line is one capacitive node (e.g. the capacitive nodes 100a, 100b and 100c in FIG. 1). In this case of FIG. 1, there are m*n capacitive nodes totally. The coupling relationship between finger and sensor grid may change the capacitance values of adjacent capacitive nodes. The detection circuit may compute the coordinate of the contact point according to the capacitive variation of the capacitive nodes on the sensor grid.
Please refer to FIG. 2. FIG. 2 is a schematic diagram illustrating a touch input device 1 and a touch sensor circuit 12 in prior art. As shown in FIG. 2, the touch input device in prior art includes a touch panel 10 and the touch sensor circuit 12 for cooperating with the panel. The touch sensor circuit 12 may include a multiplexer 120, a controller circuit 1200, a differential detection module 122 and a reference capacitor array 124. In practical circuit, the differential detection module 122 can be a comparator circuit, and the reference capacitor array 124 can be formed by multiple capacitors arranged in a specific way. Each capacitor in the reference capacitor array 124 represents a reference capacitance corresponding to a different location on the touch panel 10.
On different spot of the capacitive touch panel, each capacitive node may have different intrinsic capacitance value, which is affected by material and thickness of substrate, coupling to surrounding area and distance to the frame of the touch panel. Serious distortion will happen if all capacitive nodes are compared to one identical reference value. For example, the intrinsic capacitance value of the capacitive node around the edges may be quite different from one of the capacitive node in the middle. Therefore, different reference capacitance values are needed relative to those capacitive nodes.
In practical applications, the controller circuit 1200 may control the multiplexer 120 and utilize it to select a specific X-directional conductive line (X1˜Xm) and a specific Y-directional conductive line (Y1˜Yn), so as to select one specific capacitive node. For example, the multiplexer 120 may select X-directional conductive line X3 and Y-directional conductive line Y3 for corresponding to the capacitive node 100a. Then, the differential module 122 may compare the capacitance value of the capacitive node 100a with the reference capacitor array 124. In this embodiment, the reference capacitor array 124 is used for providing the intrinsic capacitance value of the capacitive nodes on the touch panel 10. In this way, the reference capacitor array 124 needs many fixed capacitors with different capacitance values. In order to match all the capacitive nodes, it may need m*n fixed capacitors with different capacitance values at most.
On the other hand, the capacitance value of the reference capacitor array 124 must be similar to the capacitive node under estimated. For example, the capacitive nodes 100a, 100b and 100c may be 10 pF, 11 pF and 12 pF. In this case, the capacitance value of the reference capacitor array 124 must implement an array from 10 pF to 12 pF. However, to dispose a capacitor over 10 pF in an integrated circuit may bring high cost and occupy large area.
Besides, in order to elevate the touch-input preciseness on the touch panel, the total amount of capacitive nodes (m*n) contained in one singular touch panel 10 increases gradually. To implement a huge reference capacitor array 124 into the touch input device 1 is not an ideal solution for a modern application. In addition, the intrinsic capacitance value is larger in a large sized touch panel, such that the reference capacitor array 124 with higher capacitance values is required in traditional design.
Therefore, the invention discloses a touch input device and a touch sensor circuit, which is suitable for various touch input electronic system, so as to solve said problems.